Japanese Patent Publication No. 06-40704 (JP06-40704) discloses a technology relating to “a method of operating inverters in parallel.” The disclosed technology is directed to suppression of cross current flowing between the inverter units operating in parallel. The technology assumes that reference sinusoidal signals are substantially synchronized among the inverter units. In a standalone operation of the inverter units where a synchronized signal is not given to each of the inverter units and each unit is operating independently from others, parallel operation of the inverter units is not possible if the reference sinusoidal signals used in the units are not synchronized due to an error in a crystal oscillation circuit which generates the reference sinusoidal signal.
Japanese Patent Application Publication No. 2004-15923 (JP2004-15923A) titled “Power Conversion Apparatus Connecting a Plurality of Inverters in Parallel” discloses a technology relating to a dedicated signal line which allows the inverter units to share a synchronized signal. Japanese Patent Application Publication No. 2004-336856 (JP2004-336856A) titled “Uninterruptible Power Supply System” discloses a technology relating to synchronization of inverter units operating in parallel by switching a reference signal.
Japanese Patent No. 3825665 discloses a technology relating to suppression of cross current flowing between the inverter units operating in parallel, and describes under the Background Art section that the frequency accuracy of an output from each of inverter units operating in parallel decreases if the phase of an output voltage is changed according to varying effective power. In order to evenly distribute the load among the inverter units operating in parallel, it is necessary to provide a circuit, which is shared by the inverter units to distribute the varying load.
In a parallel operation of a plurality of inverter units, if a common portion or function or circuit shared by the inverter units fails to operate properly, all of the inverter units operating in parallel do not work properly, thereby causing an issue with lowered reliability. Switching operation may solve the issue, but the operation as a whole will become complicated, which may in turn bring about an issue with lowered reliability.